Formulations for production of reaction injection molded elastomer polyurethane products, such as those widely employed in the automotive industry, generally comprise a high molecular weight polyether polyol and an organic polyisocyanate together with volatile organic blowing agents and catalyts which may be tertiary amine, organo-tin compound, or more generally a co-catalyst combination of tertiary amine and tin catalysts. Triethylene diamine catalyst, more often together with a smaller amount of tin co-catalyst, have been widely used in polyurethane RIM formulations. In addition to the usual components common to formulations for blown polyurethane products, those employed in the formation of urethane elastomeric products by reaction injection molding, typically contain chain extenders, such as low molecular weight diols (e.g. ethylene glycol or butane diol) and diamine (such as diethyl toluenediamine). The obtained RIM polymers contain the chemical bond formation of urethane, urea and to a lesser extent allophonate and biuret, in which the polymer product comprises a build-up of hard and soft block segments having the desired physical properties. This segmentation is controlled by the selection of reactants as well as by the relative reaction rate between the polyol and isocyanate. The level of catalysts and the type thereof has an important influence on the reaction rates.
To obtain good processability of a RIM part, the reactants should remain fluid in the initial stages for good flowability into the mold, but be viscous enough to prevent excess wetting of the mold surface and avoid air entrapment. While this can be achieved to greater or less extent by tin catalysts alone or by ditertiary amine catalyst such as triethylenediamine, alone, the combination of these has been found beneficial because of the remarkable synergism displayed.
When acrylic or melamine-based coatings of very high solids content above a particular level are applied to RIM products, it was found that frequently a tacky or soft finish was had. This was observed particularly in those instances when the high solids coating was applied to products produced from a RIM formulation containing a relatively large amount of tertiary amine catalyst. Since the acrylic and melamine based coatings used are cured by acid, the resulting tacky finish was attributed to the presence of residual amine in the molded polyurethane product, which interfered with cross-linking reactions needed for complete cure of the coating polymer resin. In the case of RIM formulations containing less than 0.15 parts of triethylenediamine catalyst per hundred parts of polymer (php), no adverse effects were observed on the surfaces coated with the very high solids enamels. Complete cures with no loss in physicals were obtained. Also, it was found that the incomplete curing of such higher solids coatings did not occur when the RIM formulation comprised reactive hydroxyalkyl amine catalysts (with less than 0.15 php of non-reactive tertiary amine). In that instance, as the elastomer is cured the reactive amine apparently is chemically bound to the urethane polymer backbone, preventing the amine from interfering with cure of the coating.
In investigations leading to the present invention numerous approaches were considered and pursued to overcome the problem of incomplete cure of acid-catalyzed resin coatings of high solids content applied to molded urethane products such as those produced by the RIM procedure. It was found that the problem presented could be overcome and acceptable acid-catalyzed resin coatings of high solids content could be obtained on urethane surfaces containing volatile amines, by the inclusion of certain scavenger compounds in the RIM formulation.